Method for power save mode operation in wireless local area network and apparatus for the same

ABSTRACT

A method for a power save mode operation in a wireless local area network (WLAN) system is provided. The method comprising determining a type of a receiving station (STA) indicator on the basis of the number of STAs which are associated with the AP and which intend to transmit buffered traffic; generating a traffic indicator map (TIM) element including information which indicates the receiving STA indicator and the type of the receiving STA indicator, transmitting the TIM element, receiving a poll frame for requesting data frame transmission from one STA among the STAs and transmitting the data frame to the STA.

This Application is a 35 U.S.C. §371 National Stage Entry ofInternational Application No. PCT/KR2011/007502, filed Oct. 11, 2011,and claims the benefit of U.S. Provisional Application Nos. 61/420,328,filed Dec. 7, 2010 and 61/424,678, filed Dec. 20, 2010, all of which areincorporated by reference in their entirety herein.

TECHNICAL FIELD

The present invention relates to a wireless local area network (WLAN)system, and more particularly, to a method for a power save modeoperation of a station (STA).

BACKGROUND ART

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate.

The WLAN system supports an active mode and a power save mode as anoperation mode of a station (STA). The active mode implies an operationmode in which the STA operates in an awake state capable of transmittingand receiving a frame. On the other hand, the power save mode issupported for power saving of an STA which does not require the activestate to receive the frame. An STA supporting the power save mode (PSM)can avoid unnecessary power consumption by operating in a doze mode whenit is not a time duration in which the STA can access to its radiomedium. That is, the STA operates in the awake state only for a timeduration in which a frame can be transmitted to the STA or a timeduration in which the STA can transmit the frame.

In the WLAN system, an access point (AP) manages traffic to betransmitted to STAs that operate in the power save mode. When there isbuffered traffic to be transmitted to a specific STA, a method isrequired in which the AP reports this to the STA and transmits a frame.Further, when the STA operates in the doze state, a method is requiredin which, if there is buffered traffic for the STA, the STA transitionsto the awake state to be able to normally receive the frame.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method of operating a station (STA) ina power save mode in a wireless local area network (WLAN) system and anapparatus supporting the method.

Solution to Problem

In an aspect, a method performed by an access point for a power savemode operation in a wireless local area network (WLAN) system isprovided. The method comprising determining a type of a receivingstation (STA) indicator on the basis of the number of STAs which areassociated with the AP and which intend to transmit buffered traffic;generating a traffic indicator map (TIM) element including informationwhich indicates the receiving STA indicator and the type of thereceiving STA indicator, transmitting the TIM element, receiving a pollframe for requesting data frame transmission from one STA among the STAsand transmitting the data frame to the STA.

If the number of STAs is greater than or equal to a specific threshold,the type of the receiving STA indicator may be a bit sequence type inwhich the STAs include respective association identifiers (AIDs) asidentifiers assigned when associating with the AP.

If the number of STAs is less than the specific threshold, the type ofthe receiving STA indicator may be a bitmap type in which each of bitorders corresponding to the AIDs of the STAs is set to a specific value.

In the receiving STA indicator of the bit sequence type, bits of which abit order is less than a minimum AID among the AIDs may be omitted, andthe TIM element may further include an offset indicator indicating thenumber of omitted bits.

The TIM element may be transmitted by being included in a beacon frametransmitted periodically by the AP.

The method may further include receiving, by the AP, an acknowledgement(ACK) frame from the STA in response to the data frame.

In another aspect a wireless apparatus is provided. The apparatusinclude a transceiver for transmitting and receiving a data frame and aprocessor operating by being operably coupled to the transceiver. Theapparatus is configured for determining a type of a receiving STAindicator on the basis of the number of STAs which are associated withthe AP and which intend to transmit buffered traffic, generating a TIMelement including information which indicates the receiving STAindicator and the type of the receiving STA indicator, transmitting theTIM element, receiving a poll frame for requesting data frametransmission from one STA among the STAs and transmitting the data frameto the STA.

In still another aspect, a method performed by an AP for a power savemode operation in a WLAN system is provided. The method includesswitching an operation state to an awake state to receive a TIM elementincluding a receiving STA indicator, receiving the TIM element from theAP, determining whether there is buffered traffic for the STA on thebasis of the receiving STA indicator, requesting the AP to transmit adata frame when there is the buffered traffic for the STA and receivingthe data frame from the AP after receiving of the poll frame.

The TIM element may further include type indication information of thereceiving STA indicator and the type indication information of thereceiving STA indicator may indicate either a bit sequence type in whichthe STAs having the buffered traffic include respective AIDs asidentifiers assigned when associating with the AP or a bitmap type inwhich bit values are set to specific values when bit orders correspondto the AIDs.

The method may further include, in the determining of whether there isthe buffered traffic, if the type indication information indicates thebit sequence type, determining that there is the buffered traffic forthe STA when the receiving STA indicator includes an AID of the STA.

The method may further include, in the determining of whether there isthe buffered traffic, if the type indication information indicates thebitmap type, determining that there is the buffered traffic for the STAin a case where a bit value is set to the specific value when a bitorder corresponds to the STA's AID included in the receiving STAindicator.

The method may further include, after the receiving of the data frame:

transmitting an ACK frame to the AP in response to the data frame andswitching an operation state to a doze state.

The method may further include, if there is no buffered traffic for theSTA, switching an operation state to a doze state.

Advantageous Effects of Invention

When providing a traffic indication map (TIM) element indicating whetherthere is buffered traffic for a station (STA) that operates in a dozemode, a type of the TIM element is determined on the basis of the numberof STAs having buffered traffic, and the TIM element generated in thismanner is provided. A size of traffic indication information included inthe TIM element can be optimized, thereby being able to avoidunnecessary power consumption of an STA that operates in a power savemode.

When the size of the traffic indication information included in the TIMelement is optimized, a time for occupying a radio medium by the STAthat operates in the power save mode and an access point (AP) thatmanages a power save mode operation of the STA can be reduced, and thusoverall throughput of a wireless location area network (WLAN) can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 shows an example of a power management operation.

FIG. 3 shows an example of a TIM element format.

FIG. 4 shows an example of a traffic control field and a trafficindication field according to an embodiment of the present invention.

FIG. 5 shows an example of a response procedure of an AP in a TIMprotocol.

FIG. 6 shows another example of a response procedure of an AP in a TIMprotocol.

FIG. 7 shows a procedure of a TIM protocol based on a DTIM.

FIG. 8 shows an example of a TIM element format according to anembodiment of the present invention.

FIG. 9 shows a method for a power management operation according to anembodiment of the present invention.

FIG. 10 is a block diagram showing a wireless apparatus according to theembodiments of the present invention may be implemented.

MODE FOR THE INVENTION

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, A WLAN system includes one or more Basic ServiceSet (BSSs). The BSS is a set of stations (STAs) which can communicatewith each other through successful synchronization. The BSS is not aconcept indicating a specific area

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STA5, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a plurality of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to as a mobile terminal, a wireless device, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, or simply a user. For convenience ofexplanation, the non-AP STA will be hereinafter referred to the STA.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between STAs in an infrastructure BSS including the AP isperformed via the AP in principle, the STAs can perform directcommunication when a direct link is set up. The AP may also be referredto as a central controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe inter-connected by the use of the DS. An extended service set (ESS)is a plurality of BSSs connected by the use of the DS. APs and/or STAsincluded in the ESS can communicate with each another. In the same ESS,an STA can move from one BSS to another BSS while performing seamlesscommunication.

In FIG. 1, an association ID (AID) can be assigned to each of STAs 21,22, 23, 24, and 25 while the STAS are associated with an AP 10. The AIDis used uniquely in one BSS. For example, in a current WLAN system, theAID can be given to any one of values 1 to 2007. In this case, for theAID, 14 bits can be assigned to a frame transmitted by the AP and/or theSTA, and the AID value can be given to up to 16383. In this case, 2008to 16383 may be reserved.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an AP and/or an STA sensesa wireless channel or medium before starting transmission. As a resultof sensing, if it is determined that the medium is in an idle status,frame transmission starts by using the medium. Otherwise, if it issensed that the medium is in an occupied status, the AP and/or the STAdoes not start its transmission but sets and waits for a delay durationfor medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the AP and/or the STA directlysenses the medium. The virtual carrier sensing is designed to compensatefor a problem that can occur in medium access such as a hidden nodeproblem. For the virtual carrier sending, the MAC of the WLAN systemuses a network allocation vector (NAV). The NAV is a value transmittedby an AP and/or an STA, currently using the medium or having a right touse the medium, to anther AP or another STA to indicate a remaining timebefore the medium returns to an available state. Therefore, a value setto the NAV corresponds to a period reserved for the use of the medium byan AP and/or an STA transmitting a corresponding frame.

An IEEE 802.11 MAC protocol, together with a DCF, provides a HybridCoordination Function (HCF) based on a Point Coordination Function (PCF)in which a reception AP or a reception STA or both periodically poll adata frame using the DCF and a polling-based synchronous access scheme.The HCF includes Enhanced Distributed Channel Access (EDCA) in which aprovider uses an access scheme for providing a data frame to a number ofusers as a contention-based scheme and HCF Controlled Channel Access(HCCA) employing a non-contention-based channel access scheme employinga polling mechanism. The HCF includes a medium access mechanism forimproving the Quality of Service (QoS) of a WLAN and can transmit QoSdata both in a Contention Period (CP) and a Contention-Free Period(CFP).

Meanwhile, if channel sensing is always performed for frame transmissionand reception, it causes persistent power consumption of the STA. Sincepower consumption in a reception state is not much different from powerconsumption in a transmission state, if the reception state needs to becontinuously maintained, relatively great power consumption is generatedin an STA that operates by using a battery. Therefore, when the STAsenses a channel by persistently maintaining a reception standby statein a WLAN system, ineffective power consumption may be caused without aspecial synergy effect in terms of a WLAN throughput, and thus it may beinappropriate in terms of power management.

To compensate for the problem above, the WLAN system supports a powermanagement (PM) mode of the STA.

A power management (PM) mode of a STA is classified into an active modeand a power save (PS) mode in a WLAN system. Basically, the STA operatesin the active mode. When operating in the active mode, the STA canoperate in an awake state so that a frame can be received all the time.

When operating in the PS mode, the STA operates by transitioning betweena doze state and the awake state. When operating in the doze state, theSTA operates with minimum power, and does not receive a radio signal,including a data frame, transmitted from an AP. In addition, the STAoperating in the doze state does not perform channel sensing.

The longer the STA operates in a doze state, the less the powerconsumption is, and thus the longer the STA operates. However, since aframe cannot be transmitted and received in the doze state, the STAcannot operate long unconditionally. If the STA operating in the dozestate has a frame to be transmitted to the AP, the STA can transition toan awake state to transmit the frame. However, if the AP has a frame tobe transmitted to the STA operating in the doze state, the STA cannotreceive the frame and cannot know that there is the frame to bereceived. Therefore, the STA may need to know whether there is the frameto be transmitted to the STA, and if the frame exists, may require anoperation for transitioning to the awake state in accordance with aspecific period. According to this operation, the AP can transmit theframe to the STA. This will be described with reference to FIG. 2.

FIG. 2 shows an example of a power management operation.

Referring to FIG. 2, an AP 210 transmits a beacon frame to STAs in a BSSin accordance with a specific period (step S210). The beacon frameincludes a traffic indication map (TIM) information element. The TIMelement includes information for reporting that the AP 210 has bufferedtraffic for which the STAs associated with and a frame will betransmitted. Examples of the TIM element include a TIM used to report aunicast frame and a delivery traffic indication map (DTIM) used toreport a multicast or broadcast frame.

The AP 210 transmits the DTIM one time whenever a beacon frame istransmitted three times.

An STA1 221 and an STA2 222 are STAs operating in a PS mode. The STA1221 and the STA2 222 can be configured such that they can transitionfrom a doze state to an awake state in every wakeup interval of aspecific period to receive the TIM element transmitted by the AP 210.

A specific wakeup interval can be configured such that the STA1 221transitions to the awake state in every beacon interval to receive theTIM element. Therefore, the STA1 221 transitions to the awake state(step S221) when the AP 210 transmits a first beacon frame (step S211).The STA1 221 receives the beacon frame and acquires the TIM element. Ifthe acquired TIM element indicates that there is a frame to betransmitted to the STA1 221, then the STA1 221 transmits to the AP 210 aPS poll frame that requests the AP 210 to transmit a frame (step S221a). The AP 210 transmits the frame to the STA1 221 in response to the PSpoll frame (step S231). Upon completion of frame reception, the STA1 221operates by transitioning back to the doze state.

When the AP 210 transmits a second beacon frame, a medium is busy, thatis, another device accesses to the medium for example. Thus, the AP 210may not be able to transmit the beacon frame in accordance with acorrect beacon interval but may transmit it at a delayed time point(step S212). In this case, the STA1 221 switches its mode to the wakestate in accordance with the beacon interval, but cannot receive thebeacon frame transmitted with delay, and thus transitions back to thedoze state (step S222).

When the AP 210 transmits a third beacon frame, the beacon frame mayinclude a TIM element which is configured as a DTIM. However, since themedium is busy, the AP 210 transmits the beacon frame with delay (stepS213). The STA1 221 operates by transitioning to the awake state inaccordance with the beacon interval, and can acquire the DTIM by usingthe beacon frame transmitted by the AP 210. The DTIM acquired by theSTA1 221 indicates that there is no frame to be transmitted to the STA1221 and there is a frame for another STA. Therefore, the STA1 221operates by transitioning back to the doze state. After transmitting thebeacon frame, the AP 210 transmits the frame to a corresponding STA(step S232).

The AP 210 transmits a fourth beacon frame (step S214). However, sincethe STA1 221 cannot acquire information indicating that there isbuffered traffic for the STA1 221 by receiving the TIM element twotimes, the STA1 221 may regulate a wakeup interval for receiving the TIMelement. Alternatively, if signaling information for regulating a wakeupinterval value of the STA1 221 is included in the beacon frametransmitted by the AP 210, the wakeup interval value of the STA1 221 maybe regulated. Instead of transitioning an operation state for everybeacon interval to receive the TIM element, the STA1 221 can beconfigured in the present embodiment such that the operation state istransitioned one time for every three beacon intervals. Therefore, theSTA1 221 cannot acquire a corresponding TIM element since the AP 210transmits the fourth beacon frame (step S214), and maintains the dozestate when a fifth beacon frame is transmitted (step S215).

When the AP 210 transmits a sixth beacon frame (step S216), the STA1 221operates by transitioning to the awake state, and acquires the TIMelement included in the beacon frame (step S224). The TIM element is aDTIM that indicates existence of a broadcast frame, and thus the STA1221 receives the broadcast frame transmitted by the AP 210 (step S234)instead of transmitting a PS poll frame to the AP 210.

Meanwhile, the wakeup interval assigned to the STA2 222 may have alonger period than that of the STA1 221. Therefore, the STA2 222 canreceive the TIM element by transitioning to the awake state (step S225)when the fifth beacon frame is transmitted (step S215). The STA2 222knows existence of a frame to be transmitted to the STA2 222 by usingthe TIM element, and transmits a PS poll frame to the AP 210 to requesttransmission (step S225 a). The AP 210 transmits a frame to the STA2 222in response to the PS poll frame (step S233).

In order to operate the PS mode of FIG. 2, the TIM element includes aTIM that indicates whether there is a frame to be transmitted to the STAor a DTIM that indicates whether there is a broadcast/multicast frame.The DTIM may be implemented by configuring a field of the TIM element.

FIG. 3 shows an example of a TIM element format.

Referring to FIG. 3, a TIM element 300 includes an element ID field 310,a length field 320, a DTIM count field 330, a DTIM period field 340, atraffic control field 350, and a traffic indication field 360.

The element ID field 310 is a field indicating that a correspondinginformation element is a TIM element. The length field 320 indicates atotal length including this field and its subsequent fields. A maximumvalue may be 255, and its unit may be set to an octet value.

The DTIM count field 330 informs whether a current TIM element is aDTIM. If it is not the DTIM, the DTIM count field 330 indicates thenumber of remaining TIM elements until the DTIM is transmitted. The DTIMperiod field 340 indicates a period according to which the DTIM istransmitted. The DTIM transmission period may be set to a multiple ofthe number of times of transmitting a beacon frame.

The traffic control field 350 and the traffic indication field 360indicate whether there is buffered traffic or pending traffic for aspecific STA. A 1st bit of the traffic control field 350 indicateswhether there is a multicast/broadcast frame to be transmitted. Theremaining bits are set to indicate an offset value for interpreting thesubsequent traffic indication field 360.

The traffic indication field 360 is set to a value that indicateswhether there is a frame to be transmitted to each STA. This may be setin a bitmap format in which a bit value corresponding to an AID value ofthe specific STA is set to 1. According to an AID order, bits can beassigned from 1 to 2007 in sequence. For example, if a 4^(th) bit is setto 1, it implies that traffic to be transmitted to an STA having an AIDof 4 is buffered in an AP.

Meanwhile, when setting a bit sequence of the traffic indication field360, it may be ineffective to use all bit sequences that constitute abitmap in a situation where there are many consecutive zero bits. Forthis, offset information for the traffic indication field 360 may beincluded in the traffic control field 350.

FIG. 4 shows an example of a traffic control field and a trafficindication field according to an embodiment of the present invention.

Referring to FIG. 4, a bitmap sequence that constitutes a trafficindication field 360 indicates whether there is buffered traffic for anSTA having an AID corresponding to a bitmap index thereof. The bitmapsequence constitutes indication information for AIDs from 0 to 2007.

The bitmap sequence can be configured such that 0 is set consecutivelyfrom an initial bit to a k^(th) bit. In addition, the bitmap sequencecan be configured such that 0 is set consecutively from an i^(th) bit toa last bit. This indicates that there is no buffered traffic for each ofSTAs assigned with AIDs 0 to k and each of STAs assigned with AIDs i to2007. As such, a size of the TIM element can be decreased in such amanner that offset information is provided for consecutive zerosequences from 0 to k located in a first portion of the bitmap sequenceand by omitting consecutive zero sequences located in a last portionthereof.

For this, a traffic control field 350 may include a traffic offsetsubfield 351 including offset information of consecutive zero sequencesof the bitmap sequence. The traffic offset subfield 351 can be set toindicate k. The traffic indication field 360 can be set to include bitsfrom a (k+1)^(th) bit to a (i−1)^(th).

FIG. 5 shows an example of a response procedure of an AP in a TIMprotocol.

Referring to FIG. 5, an STA 520 switches its operation state from a dozestate to an awake state to receive a beacon frame including a TIM froman AP 510 (step S510). The STA 520 interprets a received TIM element andthus can know whether there is buffered traffic to be delivered to theSTA 520.

The STA 520 contends with other STAs to access to a medium fortransmitting a PS poll frame (step S520), and transmits the PS pollframe to request the AP 510 to transmit a data frame (step S530).

Upon receiving the PS poll frame transmitted by the STA 520, the AP 510transmits a data frame to the STA 520. The STA2 520 receives the dataframe, and transmits an acknowledgment (ACK) frame to the AP 510 inresponse thereto (step S550). Thereafter, the STA2 520 switches itsoperation mode back to the doze state (step S560).

Instead of immediate response of FIG. 5 in which the data frame istransmitted immediately after receiving the PS poll frame from the STA,the AP may transmit data at a specific time point after receiving the PSpoll frame.

FIG. 6 shows another example of a response procedure of an AP in a TIMprotocol.

Referring to FIG. 6, an STA 620 switches its operation state from a dozestate to an awake state to receive a beacon frame including a TIM froman AP 610 (step S610). The STA 620 interprets a received TIM element andthus can know whether there is buffered traffic to be delivered to theSTA 620.

The STA 620 contends with other STAs to access to a medium fortransmitting a PS poll frame (step S620), and transmits the PS pollframe to request the AP 610 to transmit a data frame (step S630).

If the AP 610 receives the PS poll frame but fails to prepare for a dataframe during a specific time interval such as a short inter-frame space(SIFS), instead of directly transmitting the data frame, the AP 610transmits an ACK frame to the STA 620 (step S640). This is acharacteristic of a deferred response which is different from step S540of FIG. 5 in which the AP 510 directly transmits the data frame to theSTA 520 in response to the PS poll frame.

The AP 610 performs contending when the data frame is prepared aftertransmitting the ACK frame (step S650), and transmits the data frame tothe STA 620 (step S660).

The STA 620 transmits an ACK frame to the AP 610 in response to the dataframe (step S670), and switches its operation mode to the doze state(step S680).

When the AP transmits a DTIM to the STA, a subsequent procedure of a TIMprotocol may differ.

FIG. 7 shows a procedure of a TIM protocol based on a DTIM.

Referring to FIG. 7, an STA 720 switches its operation state from a dozestate to an awake state to receive a beacon frame including a TIM froman AP 710 (step S710). The STAs 720 can know that a multicast/broadcastframe will be transmitted by using the received DTIM.

After transmitting a beacon frame including the DTIM, the AP 720transmits the multicast/broadcast frame (step S720). After receiving themulticast/broadcast frame transmitted by the AP 710, the STAs 720 switchthe operation state back to the doze state.

Meanwhile, machine to machine (M2M) is drawing attention recently as anext generation communication technique. A standardization work isongoing to support a WLAN communication protocol supported in such acommunication environment. However, the TIM protocol of FIG. 3 to FIG. 7has requirements to apply an M2M application to the WLAN.

First, when the M2M application is applied to this WLAN environment, thenumber of STAs associated with one AP may be too many. Therefore, thenumber of associated STAs may be a maximum possible supportable number(i.e., 2007) or higher, and thus a size of a traffic indication field ofthe TIM element is increased.

Second, there are many applications that support a low transfer speed inthe M2M. Therefore, when a large-sized traffic indication field istransmitted at a low speed, a significantly great transmission time isrequired and thus power consumption of STAs is increased.

Third, most of STAs that support the M2M have traffic in which a smallamount of data is exchanged periodically. Since its periodicity is verylong, the number of STAs having a frame to be received from the APduring one beacon period is very small.

Considering the above three cases, a method for effectively compressingand providing information is required in a case where the trafficindication field of a bitmap type has a great size and most of bits areset to 0. Similarly to the previous embodiment, a method in which offsetinformation is provided while omitting consecutive zero sequenceslocated in a first portion of the bitmap sequence is ineffective when asmall number of STAs have buffered traffic and a difference of assignedAIDs is great. For example, if there is buffered traffic for two STAshaving AIDs of 10 and 2000, a bitmap length is 1991 and bits between theboth ends can be all set to 0 except for the both ends. It is not muchproblematic when the number of associated STAs is small, but when thenumber of STAs is increased and thus a great AID is assigned, aprobability of occurrence of the above case is great. Dozens of STAs areassociated with the AP in a typical WLAN environment, whereas the numberof associated STAs is sharply increased when M2M is supported.Therefore, a TIM protocol considering such a characteristic is required.

In a case where a plurality of STAs are associated with the AP and asmall number of STAs have buffered traffic, instead of using the oldbitmap-type traffic indication filed included in the TIM element, atraffic indication field including information that directly indicatesan AID of a corresponding STA may be used. In the aforementionedembodiment, if the bitmap type is used, 1991 bits are required from 10to 2000, whereas in case of directly signaling the AID, only 4 bytes arerequired since the AID is 2 bytes. That is, the less the STAs to bereported, the more effective the method of directly reporting the AID.

Meanwhile, in the TIM protocol based on the method of directly reportingthe AID, the size of the traffic indication field is linearly increasedin proportion to the number of STAs having buffered traffic. On theother hand, in case of the bitmap-type traffic indication field, since asize thereof is determined by a minimum AID value and a maximum AIDvalue, the bit size is no longer increased even if an AID value betweenthe maximum and minimum AID values is additionally reported. Therefore,a probability that the size of the traffic indication field is increasedis low even if the number of STAs is increased.

Since the size of the traffic indication field is variable depending ona type of information included in the traffic indication field, a methodin which the AP can selectively use an information type according to agiven wireless environment will be proposed. In this case, the format ofthe TIM element proposed in the previous embodiment can be used bymaintaining the format to the maximum extent possible.

A type indication field indicating the information type included in thetraffic indication field may be additionally included in the TIMelement. The type indication field may be a 1 bit field. For example, ifit is set to ‘0’, the traffic indication field is set to directlyindicate an AID, and if it is set to ‘1’, the traffic indication fieldis set in the bitmap type. The STA can interpret a type indication fieldvalue to confirm types of a traffic control field and a trafficindication field.

The type indication field can be implemented by using a reserved valueof the DTIM period field of the TIM element of FIG. 2. Subsequentoperations of the TIM protocol are not defined when a DTIM period fieldvalue is set to 0. Therefore, if the DTIM period field value is 0, itmay imply that the traffic indication field directly indicates the AID,and if the DTIM period field value is set to another value, it may implythat the traffic indication field is set in the bitmap type. If the DTIMperiod field is set to a value other than 0, it can be interpreted suchthat the DTIM period field indicates the DTIM period similarly to theconventional case. However, if the DTIM field period is 0, there is aneed to add a field for additionally reporting the DTIM period. The TIMelement may have the format of FIG. 8 when using the method of directlyreporting the AID by considering the above aspect.

FIG. 8 shows an example of a TIM element format according to anembodiment of the present invention.

Referring to FIG. 8, a TIM element 800 includes an element ID field 810,a length field 820, a DTIM count field 830, a DIM period field 840, atraffic control field 850, a DTIM period for direct notification field860, and a traffic indication field 870.

The DIM period field 840 is set to a value ‘0’ which indicates that thetraffic indication field 870 includes information directly indicating anAID of an STA.

A 1^(st) bit of the traffic control field 850 is used as informationindicating whether there is a multicast/broadcast frame. Thereafter, theremaining bits used to indicate an offset can be set to 0 since they aremeaningless when using a method of directly indicating the AID of theSTA.

The DTIM period for direct notification field 860 is set to indicate anactual DTIM period.

The traffic indication field 870 is set such that AIDs of STAs havingbuffered traffic are included one by one with a length of 2 bytes.

A method for a power management operation based on the TIM protocolaccording to the aforementioned proposed embodiment will be describedbelow with reference to FIG. 9.

FIG. 9 shows a method for a power management operation according to anembodiment of the present invention.

Referring to FIG. 9, an AP 910 analyzes AIDs of STAs for currentlybuffered traffic before generating a TIM element of a beacon frame (stepS910). A traffic indication field type is determined based on a bitmapsequence size which is a difference between a minimum AID and a maximumAID on the basis of the number of AIDs having buffered traffic.

For example, if the bitmap sequence size is greater than a valueobtained by multiplying the number of AIDs by 16, it is more effectiveto directly report the AID. Therefore, LSB 7 bits of a DTIM period fieldand a traffic control field are set to 0, and a DTIM period value is setin the DTIM period for direct notification field. The traffic indicationfield is a bit sequence when each of AIDs of STAs which are targets offrame transmission has 2 bytes, and can be configured such that bitsequences are consecutive to each other.

On the contrary, if the bitmap sequence size is less than the valueobtained by multiplying the number of AIDs by 16, it is more effectiveto transmit the bitmap-type traffic indication field by including itinto the TIM.

Upon analyzing the AIDs of the STAs, the AP 910 generates a TIM elementincluding a traffic indication field having a type determined in stepS910, and transmits the generated TIM element to an STA 920 by carryingthe generated TIM element on a beacon frame (step S920).

The STA 920 switches its operation state to an awake state to receivethe beacon frame at a predetermined time (step S930), and receives theTIM element transmitted by the AP 920 (step S920). The STA 920 caninterpret the traffic indication field included in the TIM element todetermine whether there is a frame to be transmitted to the STA 920.

If the traffic indication field is a type that includes the bufferedSTA's AID, the STA 920 can know whether there is a frame to betransmitted to the STA 920 according to whether the AID assigned to theSTA 920 is included in the traffic indication field. Taking the TIMelement format of FIG. 8 for example, if the DTIM period field value is0, the traffic control field 850 is used to determine only whether thereis a multicast/broadcast frame, and the DTIM period can be stored bybeing detected from the DTIM period for direct notification field 860.To determine whether its AID is included, a bit sequence constitutingthe subsequently transmitted traffic indication field 870 can beanalyzed by dividing it by 2 bytes. If its AID is not included, theoperation mode can be directly switched to the doze state.

If the traffic indication field is a bitmap type, the STA 920 caninterpret an offset value included in the traffic control field andbitmap information included in the traffic indication field, and thuscan know whether there is a frame to be transmitted to the STA 920according to a bit value matched to an AID assigned to the STA 920.Taking the TIM element format of FIG. 8 for example, if a value that isset in the DTIM period field 850 is not 0, the value is stored as a DTIMperiod value. The traffic control field 860 can be used to know whethera multi-cast/broadcast frame exists and to acquire offset information ofa bitmap sequence constituting the traffic indication field 880. Wheninterpreting the traffic indication field 880, a full bitmap sequencecan be interpreted by adding zeros as many as indicated by the offsetinformation included in the traffic control field 860 located before thebitmap sequence, and it can be known whether there is a frame to betransmitted. If a bit order corresponding to its AID is set to 0, theoperation mode can be switched to the doze state.

When it is confirmed that there is a frame to be transmitted to the STA920, the STA 920 performs a contention step for medium access (stepS940). The STA 920 needs to transmit a message for requesting the AP 910to transmit a frame, and a medium access right for this can be acquiredby contending with another STA or the AP. When the medium access rightis acquired in the contending step (step S940), the STA 920 transmits aPS poll frame to request the AP 910 to transmit a data frame (stepS950).

Upon receiving the PS poll frame from the STA 920, the AP 910 transmitsa data frame to the STA 920 in response thereto (step S960). In order toreport to the AP 910 that data reception is normally performed, the STA920 transmits an ACK frame (step S970) and switches its operation stateto the doze state (step S980).

Although it is shown in FIG. 9 that the number of STAs which receive theTIM element from the AP and to which the data frame is transmitted isone, a plurality of STAs may receive the TIM elements. A process ofreceiving a data frame by the STAs from the AP can be performed insequence, and can be implemented in such a manner that each STA repeatsthe contending step S940, the PS poll frame transmission step S950, thedata frame receiving step S960, and the ACK frame transmitting step S970of FIG. 9. In addition, after receiving the PS poll frame from the STA,the AP can perform the ACK frame transmitting step S640, the contendingstep S650, and the data frame transmitting step S660 of FIG. 6.

FIG. 10 is a block diagram showing a wireless apparatus according to theembodiments of the present invention may be implemented.

Referring to FIG. 10, the wireless apparatus 1000 includes a processor1010, memory 1020, and a transceiver 1030. The transceiver 1030transmits and/or receives a radio signal and implements the physicallayer of the IEEE 802.11 standard. The processor 1010 operably coupledto the transceiver 1030 analyzes AIDs of STAs, creates a TIM element,and transmits/receives the TIM element. The processor 1010 implements aMAC layer and/or a PHY layer for implementing embodiments of the presentinvention shown in FIG. 2 to FIG. 9 for transmitting a data frame. Theprocessor 1010 can be configured such that the TIM element is created inanother type based on the AIDs of the STAs. Further, the processor 1010can be configured such that the TIM element is analyzed to determinewhether there is a data frame to be transmitted and to determine whichoperation is performed between a data frame request operation and anoperation state transitioning operation.

The processor 1010 and/or the transceiver 1030 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 1020 and may be performed by the processor 1010. The memory 1020may be located inside or outside the processor 1010, and may be coupledto the processor 1110 by using various well-known means.

The invention claimed is:
 1. A traffic indication method in a wirelesslocal area network, the method comprising: receiving, by a station, froman access point (AP), a beacon frame including a traffic indication map(TIM) element, the TIM element including a type indication field and atraffic indication field; and determining, by the station, whether thereis buffered traffic for the station at the AP based on the TIM element,wherein if the type indication field is set to a first value, thetraffic indication field includes an association identifier (AID)corresponding to the station if the AP has buffered traffic for thestation, and wherein if the type indication field is set to a secondvalue, the traffic indication field includes a bitmap including aplurality of bits, wherein one of the plurality of bits corresponds tothe station and the value of said one bit indicates whether there isbuffered traffic for the station at the AP.
 2. The method of claim 1,wherein the TIM element further includes a traffic control fieldindicating whether a multicast/broadcast frame is buffered at the AP. 3.The method of claim 1, further comprising: if it is determined thatthere is buffered traffic for the station at the AP, transmitting apower save (PS)-poll frame for requesting the buffered traffic to theAP.
 4. The method of claim 1, wherein the value of the type indicationfield is based on a number of stations associated with the AP.
 5. Themethod of claim 1, wherein the traffic indication field comprises anumber k of consecutive, leading zeros if there is no buffered trafficat the AP for each station with an AID between 0 and k, inclusive.
 6. Adevice for a wireless local area network, the device comprising: atransceiver configured to receive and transmit radio signals; and aprocessor operatively coupled with the transceiver and configured to:receive, via the transceiver, from an access point (AP), a beacon frameincluding a traffic indication map (TIM) element, the TIM elementincluding a type indication field and a traffic indication field; anddetermine whether there is a buffered traffic for the device at the APbased on the TIM element, wherein if the type indication field is set toa first value, the traffic indication field includes an associationidentifier (AID) corresponding to the device if the AP has bufferedtraffic for the device, and wherein if the type indication field is setto a second value, the traffic indication field includes a bitmapincluding a plurality of bits, wherein one of the plurality of bitscorresponds to the device and the value of said one bit indicateswhether there is buffered traffic for the device at the AP.
 7. Thedevice of claim 6, wherein the TIM element further includes a trafficcontrol field indicating whether a multicast/broadcast frame is bufferedat the AP.
 8. The device of claim 6, wherein the processor is furtherconfigured to transmit a power save (PS)-poll frame for requesting thebuffered traffic to the AP if it is determined that there is bufferedtraffic for the station at the AP.
 9. The method of claim 6, wherein thevalue of the type indication field is based on a number of stationsassociated with the AP.
 10. The method of claim 6, wherein the trafficindication field comprises a number k of consecutive, leading zeros ifthere is no buffered traffic at the AP for each station with an AIDbetween 0 and k, inclusive.
 11. A traffic indication method in awireless local area network, the method comprising: transmitting, by anaccess point (AP), a beacon frame including a traffic indication map(TIM) element, the TIM element including a type indication field and atraffic indication field; and wherein if the type indication field isset to a first value, the traffic indication field includes anassociation identifier (AID) corresponding to a station for which the APhas buffered traffic, and wherein if the type indication field is set toa second value, the traffic indication field includes a bitmap includinga plurality of bits, wherein one of the plurality of bits corresponds tothe station and wherein the value of said one bit indicates whetherthere is buffered traffic for the station at the AP.
 12. The method ofclaim 11, wherein the TIM element further includes a traffic controlfield indicating whether a multicast/broadcast frame is buffered at theAP.
 13. The method of claim 11, wherein the value of the type indicationfield is based on a number of stations associated with the AP.
 14. Themethod of claim 11, wherein the traffic indication field comprises anumber k of consecutive, leading zeros if there is no buffered trafficat the AP for each station with an AID between 0 and k, inclusive.